Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Line baking tray with baking paper.
Peel the onions. Finely dice onions, mozzarella and tomatoes. Knead in a large bowl with minced meat, breadcrumbs, 8 tbsp. water, mustard, tomato paste, herbs and eggs. Season with about 1 1⁄2 tsp salt and about 1 tsp pepper. Shape minced meat mass to roast, put it on the baking tray and roast in the hot oven for about 1 hour. Take out and let cool down.
Sort the arugula, wash and drain. Finely chop the olives for the cream. Mix with salad cream and pesto. Cut meatloaf into approx. 12 slices. Prepare sandwiches from bread, cream, rocket and roast.